It is common in the field of transport refrigeration to provide both an electric motor and an internal combustion engine, such as a Diesel engine, for selectively driving a refrigerant compressor. The electric motor is manually selected when the system is located at a terminal or other source of electrical potential, and the engine is automatically selected when an electric source is disconnected. The engine has more capacity than an electric motor, but the system must be adjusted so the electric motor will not be overloaded, and thus the extra capacity of the engine is not made available.
Transport refrigeration systems control the temperature of a load space to a selected set point temperature. The temperature of the load space is sensed by a sensor disposed either in the return air path, or in the discharge air path. As disclosed in U.S. Pat. No. 3,973,618, which is assigned to the same assignee as the present application, both a return air and discharge air sensor may be provided, with the discharge air sensor being selected when the set point selection indicates a non-frozen load, and with the return air sensor being selected when the set point selection indicates a frozen load.
Some uses of transport refrigeration systems have a preference for return air control, and some have a preference for discharge air control, regardless of the type of load being conditioned. When both a return air sensor and discharge air sensor are provided on a system where the user may select either one for any type load, the control algorithm must necessarily be set for return air control, to prevent freezing of a non-frozen or perishable load.
It would be desirable and it is the object of the present invention to optimize the performance of a transport refrigeration system of the type which is selectively operable with either an electric motor or an internal combustion engine, and which also has both discharge and return air sensors which may be selected by an operator according to preference